Update of Age and sex specific Natural mortality of the blue shark (Prionace glauca) in the North Pacific Ocean
Published as ISC/16/SHARKWG-1/06
In the last stock assessment of blue shark (Prionace glauca) in the North Pacific, age and sex-specific natural mortality (indicated as “M” hereafter) of this stock was used as the input data for Stock Synthesis model (Rice et al. 2014). In Rice and Semba (2014), the estimator by Peterson and Wroblewski (1984) and Chen and Watanabe (1986) and the growth equation by Nakano (1994) and Hsu et al. (2011) were used. The estimator of M applied is divided into weight-based (Peterson and Wroblewski 1984) and age-based (Chen and Watanabe 1986). The age and sex-specific M based on the former estimator was adopted in the stock assessment with the assumption of maximum age of 30. However, in the calculation by Rice and Semba (2014), the coefficient assigned for the dry weight (1.92) was mistakenly applied to wet weight of North Pacific blue shark, instead of that for the wet weight (1.28), which needs to be corrected in the upcoming stock assessment. While the weight-based estimator has been widely applied in the stock assessment of fishery resources, the effect of variance of weight among individual is suggested to be large compared to that of body length. For example, large difference in weight would be expected between pregnant female and adult male with same age. In this context, we show the age and sex-specific M based on the length-based method used in the stock assessment of Atlantic yellowfin tuna (Thunnus albacares) this year (Method 2 in Walter et al. 2016) as well as the modified M by Peterson and Wroblewski (1984). In addition, we calculated the age and sexspecific M based on another estimators included in the review by Kenchington (2014) for comparison. Regarding growth curve necessary for the estimation of M, von Bertalanffy growth curve (VBGC) by Nakano (1994) and Hsu et al. (2011) are used for the sake of consistency with the past estimates. In addition, the estimates based on VBGC by Fujinami et al. (2016) is also indicated for reference.